Engineers refer to the
energy required by these auxiliary systems, ranging from air conditioning
to high-tech cinema setups, as “hotel load.” On most yachts
this figure is a fraction of what is required to actually propel the boat.
On warships, cruise liners, and, in this case Octopus, hotel loads
either approach or exceed what’s required to move the boat. In the
case of Limitless, however, the reasons for using diesel-electric
power go deeper.

Limitless doesn’t
require nearly as much auxiliary power as she does propulsion power. Therefore
she employs a hybrid system that utilizes both straight diesel inboard
drives and diesel-electric drives to deliver a whopping 22,000 hp and
a reported top speed of 27.5 knots. According to Capt. Craig Tafoya, Limitless’
project manager, “It is this top speed that mandated the use of
diesel-electric. Conventional diesels [that could achieve this speed]
were so high strung that, when [at] idle speed [they] would never let
us go below 16 knots. The diesel-electric option allowed for precision
control while maneuvering (once the primary diesels were disengaged) and
provided us with redundancy—we had backup propulsion if the main
engines were lost.” Such redundancy is a huge advantage for any
yacht using diesel-electric. In an emergency the output from any number
of gensets can be combined to supply propulsion to electric motors to
get you home.

But all this comes at
a price. Because of their complexity, diesel-electric systems are heavy
and take up a lot of space. Vripak International, a Dutch naval architecture
firm, estimates the average weight penalty at two to five percent more
than a conventional system, due to the additional gensets, electric motors,
and frequency converters. However, diesel-electric does provide flexibility
in engine and genset placement. The engines no longer have to be inline
with the shafts, and gensets, and converters can be placed virtually anywhere.
This ability to position equipment at the designer’s (and owner’s)
discretion has significant advantages when it comes to trim characteristics
of the boat and the spaciousness of guest quarters. No longer does the
engine room have to occupy that critical space amidships where the beam
is widest, and heavy components can be positioned to achieve optimum running
angle.